1. Field of the Invention
The present invention relates to a switching device having a configuration in which a parallel switch is connected to a semiconductor device such as thyristors in parallel, the parallel switch opens and closes current paths by opening and closing mechanical contacts in the parallel switch.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a configuration of a conventional switching device that was disclosed in a Japanese publication No. JP-B-7/108072. In FIG. 1, reference number 10 designates a thyristor circuit comprising three pairs of thyristors 11, 12, and 13 for performing a switching operation of current paths L11, L12, and L13. Each pair of thyristors 11, 12, and 13 comprises two thyristors connected in parallel and whose connected-directions of the thyristors are in opposite directions to each other. Reference number 20 denotes parallel switches comprising contact switches 21, 22 and 23 connected to the pairs of the thyristors 11, 12, and 13 in parallel for switching the current paths L11, L12, and L13 connected to each thyristor 11, 12, and 13 in the thyristor circuit 10 by opening and closing the contact switches 21, 22, and 23. Reference number 30 indicates a control section for generating a control signal indicating opening and closing of the current paths L11, L12, and L13 to which the each pair of thyristors 11, 12, and 13 and the parallel switches 21, 22, and 23 are connected.
Next, a description will now be given of the operation of the conventional switching device shown in FIG. 1.
Although each current flowing through each of the current paths L11, L12, and L13 has a different signal phase to each other, the operation of the current is the same, Accordingly, only the operation of the current flowing through the current path L11 having one current signal phase will now be explained as a typical case.
When the current path L11 is closed, the pair of thyristors 11 enters an off state (no current flows through the pair of thyristors 11) in order to reduce the loss of the power consumption of the thyristor circuit 10. In this case, the current I2 flows through the contact switch 21 in the parallel switches 20 connected to the pair of thyristors 11 in parallel in order to eliminate the loss of a large power consumption in the thyristor circuit 10.
When the current path L11 is open (through which the current I2 flows) by the parallel switch 20 under the control of the control section 30, the thyristor circuit 10 enters an On state (or it enters an active state) and the thyristor circuit 10 controls the current flow so that the current I3 flows through the pair of thyristors 11 and to halt the current I2 flowing through the current path L11. Thereby, the current I2 flowing through the current path L11 is cut off.
The operation of the conventional switching device in the above-described state will now be explained with reference to timing charts of FIGS. 2A to 2H.
FIG. 2A is a timing chart showing a time-change of the total current flow I1 flowing through the current path L11. FIG. 2B is a waveform showing a time-change of the current I2 flowing through the current path L11. FIG. 2C is a waveform showing a time-change of the current I3 flowing through the pair of thyristors 11. FIG. 2D shows a curve of an opening operation of the contact switch 21 in the parallel switch 20. FIG. 2E shows a time-change of the state of the contact switch 21 in the parallel switch 20. FIG. 2F shows a time-change of the state of the pair of thyristors 11 in the thyristor circuit 10. FIG. 2G is a waveform of an opening control signal transferred from the control section 30 to the contact switch 21 in the parallel switch 20. FIG. 2H is a waveform of a gate signal indicating a switching operation transferred from the control section 30 to the pair of thyristors 11 in the thyristor circuit 10.
In order to obtain a state where the current path L11 is open, the control section 30 generates the opening control signal shown in FIG. 2G and transfers the opening control signal to the contact switch 21 in the parallel switch 20. In addition, the control section 30 generates the gate signal having a short time-width shown in FIG. 2H and transfers the gate signal to the pair of thyristors 11. Thereby, the pair of thyristors 11 changes to the ON state from the OFF state. In this state, the amount of current flow is determined by a ratio between an impedance of the contact switch 21 in the parallel switch 20 and an impedance of the pair of thyristors 11 (Time t0).
After the opening time-length (Time period T1) of the parallel switch 20, counted from when the opening control signal is received by the parallel switch 20, is elapsed, the contact switch 21 in the parallel switch 20 is opened. Thereby, arc discharge is generated at the contact switch 21 (Time t1). Because a resistance value of the contact switch 21 in the arc discharge is larger than that of the pair of thyristors 11, the current flows through the pair of thyristors 11 (see FIGS. 2B and 2C). After the current flow changes from the parallel switch 20 to the thyristor circuit 10 is completed (Time t2) shown in FIGS. 2B and 2E, the total current flows through the pair of thyristors 11 shown in FIGS. 2A and 2C. Then, the pair of thyristors 11 halts this current I1 flowing through the current path L11 when the thyristor 11 is open based on the gate signal transferred from the control section 30 (Time t1). In this case, the opening distance of the contact switch 21 in the parallel switch 20 is L (see FIG. 2D).
The conventional switching device having the structure described above has a problem in that the parallel switch 20 re-enters the active state even if the current path L11 is cut-off by the operation of the pair of thyristors 11. That is, it is difficult to cut-off or break the current path L11 completely in the conventional switching device because the open distance L of the contact switch 21 in the parallel switch 20 is shorter in break time (cut-off time) t3 than the magnitude of a voltage applied to the parallel switch 20 and the thyristor circuit 10 after the break (cut-off) of the current path L11, when the time-length from the receiving time (Time t0) of the opening control signal by the parallel switch 20 to the break (cut-off) completion time (Time t3) of the current path L11 is shorter.
The operation when the contact switch 21 in the parallel switch 20 re-starts will now be explained with reference to FIGS. 3A to 3F.
FIG. 3A is a timing chart showing a time-change of the total current flow I1 flowing through the current path L11. FIG. 3B is a waveform showing a time-change of the current flow I2 flowing through the contact switch 21 in the parallel switch 20. FIG. 3C is a waveform showing a time-change of the current flow I3 flowing through the pair of thyristors 11. FIG. 3D shows a curve of an opening operation of the contact switch 21 in the parallel switch 20. FIG. 3E is a waveform of an opening control signal transferred from the control section 30 to the contact switch 21 in the parallel switch 20. FIG. 3F is a waveform of a gate signal indicating a switching operation transferred from the control section 30 to the pair of thyristors 11 in the thyristor circuit 10.
When the open distance L of the contact switch 21 has no adequate distance to the voltage applied to the parallel switch 20 at the time t3 at which the current path L11 is open, an electronic breakdown happens at the contact switch 21 where arc discharge is generated. In this case, the current I1 in the current path L11 flows through the contact switch 21 in the parallel switch 20. This causes a failure of the current break of the current path L11.
The function of the parallel switch 20 when the current paths L11, L12, and L13 are open is that the current flows through the thyristor circuit 10. However, the parallel switch 20 in the conventional switching device shown in FIG. 1 has no special breakdown function to eliminate the above described case. In this case, the parallel switch 20 cannot break (or cannot cut-off) the current flow at the time T4 when the amount of the total current I1 becomes zero. Therefore the conventional switching device cannot break (or cannot cut-off) the current flow I1 as long as it is detected to enter the parallel switch 20 into the active state again. At this time, the time-length T4 of the gate signal transferred from the control section 30 to the pair of the thyristors 11 is shorter than that of the sum of the opening time-length T1 of the contact switch 21 in the parallel switch 20 and the time required to set the parallel switch 20 into the state where it is re-entered into the active state.
As described above, in the conventional switching device having the configuration shown in FIG. 1, there is the problem that the conventional switching device cannot break (or cannot cut-off) the current path L12 completely because the parallel switch 2 re-enters the active state.